The present invention relates to integrated circuit structures and fabrication methods and in particular to methods of forming ferroelectric and high-k paraelectric dielectrics simultaneously on a single thin-film layer on an integrated circuit chip.
The electric susceptibility, k, is a parameter which measures the ease of polarization of a material. Polarization measures the extent to which the positive and negative charges in a material are separated. Electrically, it looks like a capacitor where the charge stored increases as the applied voltage increases. The basis for high-k materials is a large polarizability with applied electric field. Dielectrics with values of k exceeding 50 are considered high-k dielectrics.
The capacitance of a capacitor with a given plate separation is directly proportional to k and to the surface area of the device. Thus, as k increases, the surface area of the capacitor may be decreased while still maintaining the same capacitance. Therefore, the higher the k value of the material used to construct the capacitor, the more densely the capacitors can be packed on an integrated circuit chip.
Although high-k dielectrics are useful for standard capacitors, they are impractical for non-volatile memory devices. However, the materials which are known as ferroelectrics are ideal for such devices. Ferroelectric materials are extremely varied, however, they have certain properties in common, which as the name suggests, are like electrical analogs to the magnetic properties of ferromagnetic substances. One important property is that once a ferroelectric material has been polarized in one direction by the application of an external electric field, it will hold that polarization for a long time, thus providing non-volatile storage of information. (That is, the memory will retain its information even if it is not receiving any power supply voltage.) This is referred to as hysteresis behavior. (See FIG. 7 for a diagram of the polarization of a ferroelectric as a function of applied electric field.)
Ferroelectricity usually disappears above a certain temperature called the Curie temperature, critical temperature or transition temperature. Above the transition temperature the crystal is said to be in a paraelectric state. The term paraelectric suggests an analogy with paramagnetism and implies a rapid decrease in the dielectric constant as the temperature increases.
As noted earlier, one property which all ferroelectrics possess is that of having a permanent electrical polarization. This permanent electrical polarization is a product of the fact that the energy curve for ferroelectrics has two minima. (See FIG. 6 for a diagram of the energy curves of ferroelectric materials. The material is ferroelectric below the transition temperature and is paraelectric above the transition temperature.) Each minimum corresponds to a polarized state. Because there is an energy barrier between the two minima, the polarization of the ferroelectric will not change until an applied electric field reaches a certain threshold level sufficient to raise the energy of the ferroelectric above the energy barrier.
Paraelectrics, however, have only one energy minimum. Because of this, a material in the paraelectric state does not exhibit a hysteresis curve. Therefore, a paraelectric becomes polarized when an external electric field is applied. However, unlike a ferroelectric, when the external field is removed, a paraelectric returns to its unpolarized state. See FIG. 7 and FIG. 8 to see the contrast between the polarization of a ferroelectric and a paraelectric in the presence of an applied electric field.
As the semiconductor industry moves toward lower operating voltage and higher density, new materials must be introduced to provide the requisite dielectric properties. For example, high-k materials (such as Barium Strontium Titanate xe2x80x9cBSTxe2x80x9d) are promising candidates for capacitors, and ferroelectric materials (such as SrBi2Ta2O9 xe2x80x9cSBTxe2x80x9d or Pb(Zr,Ti)O3 xe2x80x9cPZTxe2x80x9d) may be used for non-volatile memory applications.
The dielectric properties of ferroelectric crystals can be significantly altered by standard processing steps such as implantation, annealing, or composition modification. In general, the ferroelectric phase can be frustrated such that the crystal forms a high-k ( greater than 50) dielectric rather than a ferroelectric. It is also possible to alter the dielectric properties of paraelectric crystals such that a ferroelectric crystal is created from high-k crystals by standard processing steps such as Bi loading.
Since many applications require both standard capacitor elements and non-volatile memory capacitors on a single chip, it is important to identify a method by which both types of elements can be realized with a minimum of processing steps. Conventional approaches include the fabrication of separate ferroelectric and xe2x80x9cstandardxe2x80x9d capacitor elements. The present application discloses a method for the simultaneous development of non-volatile memory elements and high-k capacitors on a single chip. In addition, the present application discloses a capacitor containing a single thin-film layer which has areas of both ferroelectric states and high-k paraelectric states (See FIG. 4).
Advantages of the disclosed methods and structures include: integrating both high-k and non-volatile capacitor elements on a single chip using a minimum of processing steps. Applications for this process include: co-integration of high-density dynamic random access memory (DRAM) and ferroelectric random access memory (FeRAM) cells; voltage-tunable, large decoupling capacitors for RF along with FeRAM memory elements.